1. Field of the Invention
The present invention relates to a spin-valve thin-film magnetic element in which electrical resistance changes due to the relationship between the pinned magnetization direction of a pinned magnetic layer and the magnetization direction of a free magnetic layer which is influenced by an external magnetic field, and to a thin-film magnetic head provided with the spin-valve thin-film magnetic element. More particularly, the invention relates to a technique which can improve the rate of resistance change and which can decrease the coercive force of the free magnetic layer.
2. Description of the Related Art
A spin-valve thin-film magnetic element is one type of giant magnetoresistive (GMR) element exhibiting a giant magnetoresistance effect, and detects a recorded magnetic field from a magnetic recording medium, such as a hard disk.
The spin-valve thin-film magnetic element has a relatively simple structure among GMR elements, and has a high rate of resistance change relative to an external magnetic field, thus, the resistance changes in response to a weak magnetic field.
FIG. 11 is a sectional view of a conventional spin-valve thin-film magnetic element, viewed from a surface facing a recording medium (air bearing surface; ABS).
The spin-valve thin-film magnetic element shown in FIG. 11 is a so-called xe2x80x9ctop-typexe2x80x9d single spin-valve thin-film magnetic element in which an antiferromagnetic layer, a pinned magnetic layer, a nonmagnetic conductive layer, and a free magnetic layer, one each, are deposited.
For the spin-valve thin-film magnetic element, a magnetic recording medium, such as a hard disk, travels in the Z direction in the drawing, and a fringing magnetic field from the magnetic recording medium is directed in the Y direction.
As shown in FIG. 11, an underlying layer 121 is provided on a substrate. A free magnetic layer 125, a nonmagnetic conductive layer 124, a pinned magnetic layer 123, an antiferromagnetic layer 122, and a protective layer 127 are formed in that order on the underlying layer 121.
Hard bias layers 126 are provided on both sides, in the track width (Tw) direction, of the underlying layer 121, the pinned magnetic layer 123, the nonmagnetic conductive layer 124, and the free magnetic layer 125, and electrode layers 128 are formed on the hard bias layers 126. Reference numeral 129 represents a laminate, which has a trapezoidal cross-section, including the underlying layer 121, the free magnetic layer 125, the nonmagnetic conductive layer 124, the pinned magnetic layer 123, the antiferromagnetic layer 122, and the protective layer 127.
In the spin-valve thin-film magnetic element, the magnetization direction of the pinned magnetic layer 123 is pinned antiparallel to the Y direction.
The underlying layer 121 is composed of tantalum (Ta) or the like, and the antiferromagnetic layer 122 is composed of an IrMn alloy, an FeMn alloy, an NiMn alloy, or the like. The pinned magnetic layer 123 and the free magnetic layer 125 are composed of Co, an NiFe alloy, or the like, the nonmagnetic conductive layer 124 is composed of copper (Cu), the hard bias layers 126 are composed of a cobalt-platinum (Coxe2x80x94Pt) alloy, and the electrode layers 128 are composed of a good conductor, such as Cu. In the spin-valve thin-film magnetic element having the structure shown in FIG. 11, the free magnetic layer 125 has a layered structure including an NiFe layer 125A and a Co layer 125B which is in contact with the nonmagnetic conductive layer 124.
FIG. 12 is a sectional view of another conventional spin-valve thin-film magnetic element, viewed from a surface facing a recording medium (ABS).
The spin-valve thin-film magnetic element shown in FIG. 12 is a so-called xe2x80x9cbottom-typexe2x80x9d single spin-valve thin-film magnetic element in which an antiferromagnetic layer, a pinned magnetic layer, a nonmagnetic conductive layer, and a free magnetic layer, one each, are deposited.
For the spin-valve thin-film magnetic element, a magnetic recording medium, such as a hard disk, travels in the Z direction in the drawing, and a fringing magnetic field from the magnetic recording medium is directed in the Y direction.
The conventional spin-valve thin-film magnetic element shown in FIG. 12 includes a laminate 109 in which an underlying layer 106, an antiferromagnetic layer 101, a pinned magnetic layer 102, a nonmagnetic conductive layer 102, a free magnetic layer 104, and a protective layer 107 are deposited in that order on a substrate, a pair of hard bias layers 105, and electrode layers 108 formed on the hard bias layers 105.
The underlying layer 106 is composed of Ta or the like, and the antiferromagnetic layer 101 is composed of an NiO alloy, an FeMn alloy, an NiMn alloy, or the like. The pinned magnetic layer 102 and the free magnetic layer 104 are composed of Co, an NiFe alloy, or the like, the nonmagnetic conductive layer 103 is composed of Cu, the hard bias layers 105 are composed of a Coxe2x80x94Pt alloy, and the electrode layers 108 are composed of a good conductor, such as Cu.
Since the pinned magnetic layer 102 is formed in contact with the antiferromagnetic layer 101, an exchange coupling magnetic field (exchange anisotropic magnetic field) is produced at the interface between the pinned magnetic layer 102 and the antiferromagnetic layer 101, and the pinned magnetization of the pinned magnetic layer 102 is pinned, for example, in the Y direction in the drawing.
Since the hard bias layers 105 are magnetized in the X1 direction in the drawing, the variable magnetization of the free magnetic layer 104 is aligned in the X1 direction. Thereby, the variable magnetization of the free magnetic layer 104 and the pinned magnetization of the pinned magnetic layer 102 are perpendicular to each other.
The free magnetic layer 104 includes an NiFe sub-layer 104A and a Co sub-layer 104B which is in contact with the nonmagnetic conductive layer 103.
In the spin-valve thin-film magnetic element shown in FIG. 12, a sensing current is applied from the electrode layer 108 formed on the hard bias layer 105 to the pinned magnetic layer 102, the nonmagnetic conductive layer 103, and the free magnetic layer 104. A magnetic recording medium, such as a hard disk, travels in the Z direction in the drawing, and when a fringing magnetic field from the magnetic recording medium is applied in the Y direction, the magnetization direction of the free magnetic layer 104 is rotated from the X1 direction to the Y direction. At this stage, electrical resistance changes due to the relationship between the varied magnetization direction of the free magnetic layer 104 and the pinned magnetization direction of the pinned magnetic layer 102, which is referred to as a magnetoresistance (MR) effect, and the fringing magnetic field from the magnetic recording medium is detected by a voltage change based on the change in the electrical resistance.
With respect to the spin-valve thin-film magnetic element shown in FIG. 11, a structure has been proposed, in which a back layer composed of a nonmagnetic conductive material, such as Au, Ag, or Cu, is formed at the underlying layer 121 side of the free magnetic layer 125 so that the mean free path of spin-up electrons, that contribute to the magnetoresistive effect, is extended, that is, a so-called xe2x80x9cspin filter effectxe2x80x9d is used, thus enabling to obtain a large rate of resistance change (xcex94R/R).
However, in the structure shown in FIG. 11, when a back layer composed of Cu is added between the free magnetic layer 125 and the underlying layer 121 composed of Ta, if the Cu back layer is deposited at a thickness of approximately several ten angstroms on the Ta underlying layer 121, it is difficult to deposit the back layer of Cu with satisfactory crystal orientation, resulting in a decrease in orientation of the back layer, and thus it is difficult to obtain a large rate of resistance change (xcex94R/R). Additionally, irregularities are likely to occur on the back layer formed on the Ta underlying layer 121, and the free magnetic layer 125, the nonmagnetic conductive layer 124, the pinned magnetic layer 123, and the antiferromagnetic layer 122 formed on the irregular back layer are likely to have uneven thicknesses, and thus it is difficult to obtain a spin-valve thin-film magnetic element exhibiting a high magnetoresistance effect.
With respect to the spin-valve thin-film magnetic element shown in FIG. 12, the protective layer 107 composed of Ta is usually deposited on the free magnetic layer 104, and if the free magnetic layer 104 is composed of an NiFe alloy, a thermal diffusion of elements tends to occur at the interface between the Ta layer and the NiFe alloy layer. If the diffusion of elements is caused by heating at the interface between the free magnetic layer 104 and the protective layer 107, the magnetic thickness (Msxc2x7t) of the free magnetic layer 104 is decreased. Moreover, the dispersion of magnetic anisotropy increases in the vicinity of the interface between the protective film 107 and the free magnetic layer 104, and there is a possibility that the coercive force of the free magnetic layer 104 increases and the rate of resistance change decreases.
Additionally, although the present inventors have proposed a so-called xe2x80x9csynthetic-ferri-pinned typexe2x80x9d spin-valve thin-film magnetic element in which a pinned magnetic layer is divided by a nonmagnetic intermediate layer into a plurality of layers, there is also a possibility that the problems in the free magnetic layer as described above arise in such a structure.
Accordingly, it is an object of the present invention to provide a spin-valve thin-film magnetic element provided with a back layer, in which the crystal orientation of the back layer is improved and the rate of resistance is improved. It is another object of the present invention to provide a spin-valve thin-film magnetic element in which the coercive force of a free magnetic layer adjacent to a back layer is decreased, soft magnetic properties are improved, and sensitivity is improved.
It is another object of the present invention to provide a bottom-type spin-valve thin-film element, in which a decrease in the magnetic thickness of a free magnetic layer is prevented, an increase in the dispersion of magnetic anisotropy at the interface between the free magnetic layer and a protective layer is prevented, an increase in the coercive force of the free magnetic layer is prevented, and a decrease in the rate of resistance change is inhibited.
It is another object of the present invention to provide a thin-film magnetic head provided with the spin-valve thin-film magnetic element as described above.
In accordance with the present invention, a spin-valve thin-film magnetic element includes a substrate; an antiferromagnetic layer; a pinned magnetic layer in contact with the antiferromagnetic layer, the magnetization direction of the pinned magnetic layer being pinned by an exchange coupling magnetic field with the antiferromagnetic layer; a nonmagnetic conductive layer in contact with the pinned magnetic layer; a free magnetic layer in contact with the nonmagnetic conductive layer, the magnetization direction of the free magnetic layer being aligned in a direction perpendicular to the magnetization direction of the pinned magnetic layer; and a back layer composed of a nonmagnetic conductive material formed in contact with the free magnetic layer at the opposite side of the nonmagnetic conductive layer. The back layer is composed of at least one metal selected from the group consisting of Ru, Pt, Ir, Rh, Pd, Os, and Cr.
When the back layer is composed of the metal or the alloy described above, the back layer can easily have a satisfactory crystal orientation, and the lattice matching at the interface between the back layer and the free magnetic layer can be satisfactorily set, and thus the spin filter effect of the back layer for selecting spin-up electrons can be satisfactorily exhibited, resulting in a high rate of resistance change.
Prior to describing a reason for an increase in the rate of magnetoresistance change due to the spin filter effect, the principle of a giant magnetoresistance effect of a spin-valve thin-film magnetic element will be briefly described below.
When a sensing current is applied to a spin-valve thin-film magnetic element, conduction electrons mainly move in the vicinity of the nonmagnetic conductive layer having a small electrical resistance. Theoretically, two types of conduction electrons are present in equal quantity, namely, spin-up conduction electrons and spin-down conduction electrons.
The rate of magnetoresistance change of the spin-valve thin-film magnetic element has a positive correlation with a difference in the mean free path between the two types of conduction electrons.
The spin-down conduction electrons are always scattered at the interface between the nonmagnetic conductive layer and the free magnetic layer regardless of the direction of an applied external magnetic field, and the probability of transferring to the free magnetic layer remains low, and the mean free path of the spin-down conduction electrons remains shorter than that of the spin-up electrons.
In contrast, the spin-up conduction electrons have an increased probability of transferring from the nonmagnetic conductive layer to the free magnetic layer when the magnetization direction of the free magnetic layer is set parallel to the magnetization direction of the pinned magnetic layer by an external magnetic field, and the mean free path is increased. As the magnetization direction of the free magnetic layer is varied from a state parallel to the magnetization direction of the pinned magnetic layer due to an external magnetic field, the probability of being scattered at the interface between the nonmagnetic conductive layer and the free magnetic layer is increased and the mean free path of the spin-up conduction electrons is decreased.
As described above, the mean free path of the spin-up conduction electrons greatly changes in comparison with the mean free path of the spin-down conduction electrons, and the difference between the two mean free paths is greatly changed, and thus the resistivity is changed, and the rate of magnetoresistance change (xcex94R/R) of the spin-valve thin-film magnetic element is increased.
If the back layer is deposited on the free magnetic layer at the side opposite of the nonmagnetic conductive layer, the back layer forms a potential barrier at the interface with the free magnetic layer, thus enabling the extension of the mean free path of the spin-up conduction electrons passing through the free magnetic layer. That is, since the so-called xe2x80x9cspin filter effectxe2x80x9d can be exerted, the rate of magnetoresistance change can be further improved. The spin filter effect is reduced if the crystal orientation of the back layer becomes disordered. When the layer underlying the back layer has irregularities and waviness and the back layer itself has irregularities and unevenness, the spin filter effect is reduced. Therefore, by forming the back layer using at least one metal selected from the group consisting of Ru, Pt, Ir, Rh, Pd, Os, and Cr, matching with the underlying layer is improved, and the probability that a satisfactory crystal orientation is obtained is increased, and thus the spin filter effect of the back layer for selecting the spin-up conduction electrons is easily exhibited, resulting in a high rate of resistance change.
In the spin-valve thin-film magnetic element of the present invention, preferably, at least the back layer, the free magnetic layer, the nonmagnetic conductive layer, the pinned magnetic layer, and the antiferromagnetic layer are deposited in that order on the substrate. The magnetization direction of the pinned magnetic layer is pinned by an exchange coupling magnetic field with the antiferromagnetic layer, and the magnetization direction of the free magnetic layer is aligned in a direction perpendicular to the magnetization direction of the pinned magnetic layer.
In the top-type spin-valve thin-film magnetic element, in which the back layer, the free magnetic layer, the nonmagnetic conductive layer, and the pinned magnetic layer are deposited on the substrate, and the antiferromagnetic layer is disposed thereon, if the crystal orientation of the back layer is improved and irregularities are decreased, the irregularities of other layers formed on the back layer are decreased and unevenness is also overcome, so, the spin filter effect is easily obtained and the rate of resistance change is greatly improved.
In the spin-valve thin-film magnetic element of the present invention, preferably, at least the antiferromagnetic layer, the pinned magnetic layer, the nonmagnetic conductive layer, the free magnetic layer, and the back layer are deposited in that order on the substrate. The magnetization direction of the pinned magnetic layer is pinned by an exchange coupling magnetic field with the antiferromagnetic layer, and the magnetization direction of the free magnetic layer is aligned in a direction perpendicular to the magnetization direction of the pinned magnetic layer. In the bottom-type spin-valve thin-film magnetic element in which the antiferromagnetic layer is disposed closer to the substrate, it is also possible to obtain the spin filter effect by the back layer.
In the spin-valve thin-film magnetic element of the present invention, preferably, the back layer is deposited on the substrate with an underlying layer composed of Ta therebetween.
Since the back layer formed on the Ta underlying layer is composed of the material described above, a satisfactory crystal orientation of the back layer can be easily obtained, and it is possible to easily obtain a back layer in which irregularities are decreased and the surface roughness is decreased. Since irregularities and defects are not easily caused, and the interface between the back layer and the free magnetic layer has a satisfactory joining state, a satisfactory spin filter effect is thus exhibited.
Preferably, a protective layer is deposited on the free magnetic layer on the side opposite from the substrate, the protective layer being composed of at least one metal selected from the group consisting of Ru, Pt, Ir, Rh, Pd, Os, and Cr.
When the metal or the alloy described above is formed directly on the free magnetic layer, unlike the conventional protective layer composed of Ta, thermal diffusion does not easily occur, and thus a decrease in the magnetic thickness of the free magnetic layer can be prevented, an increase in the dispersion of magnetic anisotropy at the interface between the free magnetic layer and the protective layer can be prevented, an increase in the coercive force of the free magnetic layer can be inhibited, and a decrease in the rate of resistance change can be inhibited.
Preferably, the free magnetic layer is composed of NiFe, and the protective layer has a thermal-diffusion-inhibiting function.
When the free magnetic layer is composed of NiFe, thermal diffusion may easily occur depending on the material of the protective layer, and in particular, when the protective layer is composed of Ta, thermal diffusion is increased, and thus diffusion of elements at the interface easily affects the thin-film magnetic element if heated. By using the protective layer composed of the metal or the alloy described above having the thermal-diffusion-inhibiting function, thermal diffusion does not easily occur, a decrease in the magnetic thickness of the free magnetic layer can be prevented, an increase in the dispersion of magnetic anisotropy at the interface between the free magnetic layer and the protective layer can be prevented, an increase in the coercive force of the free magnetic layer can be inhibited, and a decrease in the rate of resistance change can be inhibited.
Preferably, the spin-valve thin-film magnetic element of the present invention further includes bias layers for aligning the magnetization direction of the free magnetic layer in the direction perpendicular to the magnetization direction of the pinned magnetic layer, the bias layers being formed on both sides of a laminate including at least the antiferromagnetic layer, the pinned magnetic layer, the nonmagnetic conductive layer, the free magnetic layer, and the back layer; and electrode layers for applying a sensing current to the laminate, the electrode layers being formed on the bias layers.
By providing the bias layers, a bias can be applied to the free magnetic layer so that the magnetization direction of the free magnetic layer can be uniformly aligned, and when the magnetization direction of the free magnetic layer is varied, the magnetization direction of the entire free magnetic layer can be changed uniformly. By providing the electrode layers, a sensing current can be applied to the thin-film magnetic element.
Preferably, the pinned magnetic layer includes a nonmagnetic intermediate sub-layer, and first and second pinned magnetic sub-layers sandwiching the nonmagnetic intermediate sub-layer, the magnetization direction of the first pinned magnetic sub-layer is antiparallel to that of the second pinned magnetic sub-layer, and the first and second pinned magnetic sub-layers are in a ferrimagnetic state.
By employing a structure in which the pinned magnetic layer includes two pinned magnetic sub-layers and by setting the magnetization directions of the two pinned magnetic sub-layers to be in a ferrimagnetic state, it is possible to balance the magnetization directions in the pinned magnetic layer, and the magnetization direction of the pinned magnetic layer can be stably maintained, and thus it is possible to obtain a thin-film magnetic element having stable output characteristics.
In the spin-valve thin-film magnetic element, a structure in which the pinned magnetic layer is divided into two sub-layers may be referred to as a so-called xe2x80x9csynthetic-ferri-pinned typexe2x80x9d, and by using such a structure, the demagnetizing field (dipole magnetic field) due to the pinned magnetization of the pinned magnetic layer is cancelled by the magnetostatic coupling magnetic field of the first pinned magnetic sub-layer and the magnetostatic coupling magnetic field of the second pinned magnetic sub-layer. Thereby, it is possible to reduce the influence of the demagnetizing field (dipole magnetic field) due to the pinned magnetization of the pinned magnetic layer on the variable magnetization direction of the free magnetic layer.
When the pinned magnetic layer is separated by the nonmagnetic intermediate sub-layer in the spin-valve thin-film magnetic element as described above, one of the pinned magnetic sub-layers fixes the other pinned magnetic sub-layer in a proper direction, and thus the pinned magnetic layer can be maintained in a very stable state.
The divided pinned magnetic layer reduces the influence of the demagnetizing field (dipole magnetic field) due to the pinned magnetization of the pinned magnetic layer on the free magnetic layer, and the variable magnetization direction of the free magnetic layer can be more easily corrected to a desired direction, and it is possible to produce a superior spin-valve thin-film magnetic element with little asymmetry, thus facilitating the control of the variable magnetization direction of the free magnetic layer.
Herein, asymmetry is defined as the degree of asymmetry of a regenerated output waveform, and if the waveform is symmetrical, the asymmetry is decreased. Therefore, as the asymmetry is brought closer to zero, the regenerated output waveform has a more superior symmetry.
The asymmetry is zero when the direction of the variable magnetization of the free magnetic layer and the direction of the pinned magnetization of the pinned magnetic layer are orthogonal to each other. When the asymmetry is greatly increased, it is not possible to read the data accurately from the media, resulting in an error. Therefore, as the asymmetry is brought closer to zero, the reliability of processing regenerated signals is improved, resulting in a superior spin-valve thin-film magnetic element.
In general, the demagnetizing field (dipole magnetic field) Hd has a nonuniform distribution in which the values are large at the ends and small in the center in the element height direction, and in some cases, the free magnetic layer may be prevented from being aligned in a single-domain state. However, by using the pinned magnetic layer including the sub-layers as described above, the dipole magnetic field Hd can be set to be substantially zero, and thus the free magnetic layer is not prevented from being aligned in a single-domain state due to the formation of domain walls, resulting in nonuniform magnetization, and thus it is possible to prevent Barkhausen noise, etc. from occurring, which may result in instability in which signals from the magnetic recording medium are inaccurately processed in the spin-valve thin-film magnetic element.
Preferably, in the present invention, the free magnetic layer includes a nonmagnetic intermediate sub-layer, and first and second free magnetic sub-layers sandwiching the nonmagnetic intermediate sub-layer, the magnetization direction of the first free magnetic sub-layer is antiparallel to that of the second free magnetic sub-layer, and the first and second free magnetic sub-layers are in a ferrimagnetic state.
By using a structure in which the free magnetic layer includes two free magnetic sub-layers and by setting the magnetization directions of the two free magnetic sub-layers to be in a ferrimagnetic state, it is possible to balance the magnetization directions in the free magnetic layer, and the magnetization direction of the free magnetic layer can be stably maintained, and thus it is possible to obtain a thin-film magnetic element having stable output characteristics.
Preferably, the antiferromagnetic layer is composed of one of an Xxe2x80x94Mn alloy and a Ptxe2x80x94Mnxe2x80x94Xxe2x80x2 alloy, where X is an element selected from the group consisting of Pt, Pd, Ir, Rh, Ru, and Os, and Xxe2x80x2 is at least one element selected from the group consisting of Pd, Cr, Ru, Ni, Ir, Rh, Os, Au, Ag, Ne, Ar, Xe, and Kr.
Since the antiferromagnetic layer composed of such an alloy has a high blocking temperature at which the exchange coupling magnetic field disappears, the antiferromagnetic layer is thermally stable, and since a high exchange coupling magnetic field for pinning the magnetization of the pinned magnetic layer is obtained, the ability of pinning the pinned magnetic layer can be increased.
More preferably, the antiferromagnetic layer is composed of an Xxe2x80x94Mn alloy, where X is an element selected from the group consisting of Pt, Pd, Ir, Rh, Ru, and Os, and the X content is in the range from 37 to 63 atomic %. Alternatively, more preferably, the antiferromagnetic layer is composed of an Xxe2x80x2xe2x80x94Ptxe2x80x94Mn alloy, where Xxe2x80x2 is at least one element selected from the group consisting of Pd, Cr, Ru, Ni, Ir, Rh, Os, Au, Ag, Ne, Ar, Xe, and Kr, and the Xxe2x80x2+Pt content is in the range from 37 to 63 atomic %.
By using the Xxe2x80x94Mn alloy or the Xxe2x80x2xe2x80x94Ptxe2x80x94Mn alloy as the antiferromagnetic layer, in comparison with an NiO alloy, an FeMn alloy, and an NiMn alloy which have been conventionally used as the antiferromagnetic layer, it is possible to obtain a spin-valve thin-film magnetic element having superior characteristics, for example, a larger exchange coupling magnetic field, a higher blocking temperature, and a higher corrosion resistance.
Preferably, a Co layer is disposed on at least one of the free magnetic layer side of the nonmagnetic conductive layer and the pinned magnetic layer side of the nonmagnetic conductive layer.
In the spin-valve structure in which the nonmagnetic conductive layer is interposed between the free magnetic layer and the pinned magnetic layer, by disposing the Co layer on both sides or one side of the nonmagnetic conductive layer, a larger change in resistance can be easily obtained.